The present invention relates to a polycarbonate molding material for optical discs. More particularly, it relates to a pelletized polycarbonate molding material useful for producing transparent substrates of optical discs which are high-density recording media, by injection molding, in which the molding material is capable of molding the optical discs at high productivity with a short molding cycle in injection molding.
Since polycarbonate resins are excellent in transparency, heat resistance and low water absorption, polycarbonate resins are widely used as raw materials in optical uses, especially, for the manufacture of transparent substrates of optical discs such as CD, CD-ROM, MO, PD, CD-R, CD-RW, DVD-ROM, DVD-R and DVD-RAM. Expansion of optical disc markets in recent years has been remarkable and the productivity thereof as well as quality and precision in the manufacture of the transparent substrates of such optical discs has been regarded as important.
In manufacture of the transparent substrates which are usually produced by injection molding, it is advantageous in view of productivity that the molding cycle time be reduced. Generally, the injection molding cycle is constituted by the steps of an injection step, a dwelling step, a cooling step and a withdrawal step of the molded product. In the manufacture of CD according to the latest process, the molding cycle is set to be not more than 4.0 seconds. The molding cycle time is variable depending on the molding conditions and other relevant factors such as capacity of the molding withdrawal device. In the case where the total molding cycle time through the steps is defined to be within 4.0 seconds, the time of the cooling step must be set to be 2.0 to 3.0 seconds. Therefore, the plasticization time of the resin comes into question. Usually, the plasticization of the resin is completed within the period of cooling which follows the steps of injection and dwelling, and the shorter the waiting period till injection, the more advantageous for a "shorter tact time" in productivity.
Reduction of the substrate cooling time by enhancing the cooling efficiency of the mold is indeed one means for attaining a "shorter tact time" in productivity where the total cycle time does not exceed 4.0 seconds. However, if the cooling time is reduced excessively, the resin plasticization time may be longer than the cooling time, so that there is caused a problem of the necessity to elongate the cooling time till resin plasticization is completed despite the fact that actual cooling of the substrate is already finished. On the other hand, if the mold is opened in order to take out the molded product when plasticization is not yet completed, the resin being plasticized may flow into the sprue of the mold (fixed) integral with the screw unit, thereby causing imperfect plasticization. Thus, the reduction of the resin plasticization period is an essential requirement. The resin plasticization time may be reduced by increasing the screw speed, but increase of screw speed tends to cause air to be easily mingled into molten resin, thereby raising the risk of inducing silverstreaks on the molded product.
Usually, plasticization and metering of the resin in the molding machine are performed by melting the resin pellets at a high temperature, uniformly plasticizing the resin by rotating the screw in the cylinder at high speed while letting it backtrack, and forwarding a measured amount of molten resin ahead of the cylinder. Thus, the shorter the period in which the resin pellets are melted, the shorter the plasticization time becomes.
As a result of the present inventors' earnest studies on the melting behavior of the resin pellets, it has been found that although the melting time of the resin pellets depends on such factors as feed zone temperature of the cylinder and screw speed, the plasticization time is greatly affected by the length and diameter of the pellets, that is, by using the pellets having more specific length and diameter distributions, the plasticization time can be reduced. Further, it has been found that since the plasticization time can be shortened by uniformalizing the size of the resin pellets, it is not necessary to rotate the screw at high speed when metering the resin, whereby it is possible to avoid causing silverstreaks due to drag-in of air. The present invention has been attained on the basis of the above finding.